Write Spike Performance Enhancement In Hybrid Storage Systems

ABSTRACT

In an embodiment, a hybrid storage array one uses two or more storage device tiers provided by solid state drives (SSDs) and hard disk drives (HDDs). Random writes are collected and written to a write cache extension, such as a portion of the SSD storage tier. The write cache extension absorbs such accesses that would otherwise be written to HDD storage directly. Data structures are created in a cache memory local to an array controller representing the location on the write cache extension to which the writes were committed and a location in the storage system where they were originally intended to go. The write cache extension can be enabled all of the time, or only when the array controller write cache experiences certain operating conditions, such as when its utilization exceeds a predetermined amount. The approach improves the overall performance of the hybrid array.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/071,557, filed Mar. 25, 2011. The entire teachings of the aboveapplication are incorporated herein by reference.

BACKGROUND

1. Field of the Disclosure

This disclosure relates generally to information handling systems anddevices, and more particularly to hybrid storage arrays.

2. Description of the Related Art

As the value and use of information continues to increase, individualsand businesses seek additional ways to process and store information.One option is an information handling system. An information handlingsystem generally processes, compiles, stores, and/or communicatesinformation or data for business, personal, or other purposes. Becausetechnology and information handling needs and requirements can varybetween different applications, information handling systems can alsovary regarding what information is handled, how the information ishandled, how much information is processed, stored, or communicated, andhow quickly and efficiently the information can be processed, stored, orcommunicated. These variations allow information handling systems to begeneral or configured for a specific user or specific use such asfinancial transaction processing, airline reservations, enterprise datastorage, or global communications. In addition, information handlingsystems can include a variety of hardware and software components thatcan be configured to process, store, and communicate information and caninclude one or more computer systems, data storage systems, andnetworking systems.

One type of information handling system is a storage array. A storagearray typically includes multiple disk drives or similar persistent,inexpensive storage units. A storage array can allow large amounts ofdata to be stored in an efficient manner while providing redundancy topromote reliability such as Redundant Array of Inexpensive Disks (RAID)functionality.

In addition to the disk drives or other persistent storage units, astorage array commonly also includes one or more array controllers(typically taking the physical form of one or more integrated circuitsor circuit boards), and interface circuits to connect the storage arrayto an external host. The host may be a personal computer, a networkserver or some other information handling system. The controllerincludes one or more processors or similar hardwired logic that causesthe storage array to read or write data to or from its persistentstorage in response to requests received from the host. The controlleralso commonly includes memory that acts as a buffer or temporary storagefor data being transferred to and from persistent storage.

Hybrid-based storage solutions combine multiple storage technologies,such as Solid State Drives (SSDs) and Hard Disk Drives (HDDs) in thesame storage array. Hybrid storage arrays are a particularly attractivesolution for large volume applications such as data centers and cloudcomputing where high performance and low cost are each of importance.These high performance hybrid arrays can combine intelligent cachingsoftware with low cost HDDs (such as Serial ATA (SATA) drives or fasterSmall Computer System Interface (SCSI) drives), and/or high-performanceenergy-efficient SSDs. The hybrid approach combines the scalability andrelative affordability of HDD technology with the low-latency I/Operformance and energy efficiency of SSDs.

Hybrid storage arrays are typically managed by a cache-pool-awarestorage controller which uses various techniques to determine how tomost efficiently provision the data between the HDD and SSD devices. Forexample, write data blocks may be addressed by an application softwareprogram to a primary rotating medium such as the HDDs. Intelligent SSDcaching software analyzes patterns in these application I/O requests andbuilds access tables. The caching software uses these access tables tomove the more frequently accessed data blocks from the HDD to the SSDtier so that faster retrieval is possible during future requests for thesame data blocks. The intelligent caching software can alsotransparently execute writes directly to the low latency SSD media sothat high frequency writes can be quickly acknowledged back to theapplication. Data written this way to the SDDs is then pushed as abackground task to the HDDs when access to it becomes less frequent.

In addition, these hybrid storage arrays can also provide other servicessuch as RAID, load balancing, encryption, and management functions whilefurther isolating the application from the underlying hardware.

SUMMARY OF THE DISCLOSURE

Existing hybrid storage solutions can thus be used to deliver improvedperformance. However, these solutions become less advantageous when anapplication enters particular modes. Such modes occur, for example, whenan application decides to flush a significant quantity of data to astorage array. Because of the nature of parity RAID schemes, every writeto the storage array, if sufficiently small and random in nature,requires multiple writes and multiple reads to complete a RAID access.If the storage array performance is already borderline sufficient, itmay become insufficient as soon as these “write spikes” hit.

The present disclosure provides a solution that includes a uniqueapproach to write acceleration in hybrid storage arrays. The approachprevents write spikes from ever hitting the slower HDD drives that theapplications are similarly trying to read, and therefore stops thosespikes from adversely affecting application performance.

In an approach which incorporates the inventive solution herein, theSSDs are assigned as a “tier-zero” level storage, and the HDDs as“tier-one” level storage. The array software determines which pagesshould reside on the SSD tier and which pages should reside on the HDDtier by measuring the frequency of read and/or write requests to themover time. The pages are preferably organized into “frequency zones”represented as “very hot”, “hot”, “warm” and “cold”. Very hot pages areaccessed so frequently that they are cached in a memory that is local tothe storage controller. Hot pages are migrated to the SSD tier from thestorage controller cache memory when space is available, or are tradedto the SSD tier in exchange for cold pages. In the absence of hot pages,warm pages can be migrated to the SSD tier or traded for cold pages bythe same rules.

In accordance with the preferred embodiment, a portion of the SSD tieris also reserved for a special function, called a write cache extension.A mechanism is provided such that random writes that were initiallyintended to be written to the HDD tier are first collected in the writecache extension portion. They then are written sequentially from thewrite cache extension to the SSD tier at a later time, such as when thenumber of random writes collected is near a RAID stripe size.

Metadata is also preferably recorded in the write cache extension thatindicates the original logical block addresses (LBAs) for which thewrites were targeted

The mechanism can be selectively enabled only when needed, such as whenthe array controller write cache exceeds a predetermined utilization,such as 70%. When this condition is met, the acceleration mechanism isenabled and begins absorbing any random writes that would otherwise bewritten to the HDD/SDD tiers directly.

As a result, a comprehensive and unique solution is provided with thecontroller software automatically 1) migrating data between thecontroller cache, tier-zero and tier-one storage, as well as 2) using areserve portion of the tier-zero storage to absorb application writespikes.

In one particular arrangement, a storage array apparatus provides atier-zero storage portion to which preferably a first relatively faststorage scheme is applied, a tier-one storage portion, to which a secondrelatively slow storage scheme is applied, and a storage cache portion.A write cache extension portion preferably resides in the tier-zeroportion or other portion with relatively fast device access time.

A storage array controller manages access to the storage cache,tier-zero and tier-one portions by migrating data between them, based ona measured demand for specific data pages. The storage array controlleralso detects particular types or particular patterns of storage commands(such as requests for small randomly ordered writes) to the tier-onestorage portion. If the storage command falls into such a pattern , thestorage controller then determines if the storage cache is experiencingsome other condition, such as if it is already full by more than apredetermined amount. If the other condition is met, the storagecontroller then stores data associated with the storage command in thewrite cache extension portion.

The storage controller also operates to migrate data from the writecache extension portion to the tier-one storage portion. As one examplein a case where the detected storage pattern is random writes, this mayoccur only after a predetermined number of the random writes have beenstored in the write cache portion.

In yet other aspects, data may be transferred from the write cacheextension only when number of random writes it has stored is near or ata full RAID stripe size as used by the tier-one storage.

In further aspects, the storage array may store metadata associated withthe storage command patterns. As one example, in a case where thedetected storage pattern is random writes, the metadata preferablyindicates the logical block addresses for which the random write(s) areeventually intended. This metadata can also be stored temporarily in thewrite cache extension portion.

The storage array controller may migrate pages between the various tiersby measuring the frequency of RAID requests, such as by organizing pagesof the array into multiple frequency zones. In one example embodiment,these frequency zones may represented as very hot, hot, and cold. Veryhot pages are cached in storage cache main portion. Hot pages aremigrated to the tier-zero portion when space is available or are tradedto the tier-zero portion in exchange for cold pages in the tier-oneportion. The frequency zones may further include a warm frequency zonewhereby, in the absence of hot pages, warm pages are migrated to thetier-zero portion in exchange for cold pages in the tier-one portion.

In typical implementations, the tier-one portion may be one or more harddisks drives and tier-zero portion may be one or more solid statedrives. However, other hybrid solutions using different storagetechnologies are possible, mixing various levels of performance andcosts of solid state, magnetic disk, optical, and/or other media types.

In a further aspect, the storage controller may be implemented assoftware code executing on a programmable data processor, as hardwiredlogic, in programmable gate arrays, or any other known means forimplementing the logic of data processing operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a high level diagram of a hybrid storage array.

FIG. 2 is a high level flow diagram illustrating how very hot, hot, warmand cold pages are migrated between various storage tiers.

FIG. 3 shows the general logic for making decisions about migrating datablocks.

DETAILED DESCRIPTION OF AN EMBODIMENT

For purposes of this disclosure, an information handling system mayinclude any instrumentality or aggregate of instrumentalities operableto compute, classify, process, transmit, receive, retrieve, originate,switch, store, display, manifest, detect, record, reproduce, handle, orutilize any form of information, intelligence, or data for business,scientific, control, or other purposes. For example, an informationhandling system may be a personal computer, a network storage device, orany other suitable device and may vary in size, shape, performance,functionality, and price. The information handling system may includerandom access memory (RAM), one or more processing resources such as acentral processing unit (CPU) or hardware or software control logic,ROM, and/or other types of nonvolatile memory. Additional components ofthe information handling system may include one or more disk drives, oneor more network ports for communicating with external devices as well asvarious input and output (I/O) devices, such as a keyboard, a mouse, anda video display. The information handling system may also include one ormore buses operable to transmit communications between the varioushardware components.

FIG. 1 shows an information handling system that implements a hybridstorage array 100 as one embodiment more fully described below. In thetypical environment, the storage array 100 provides a data storagefunction to a host computer 200. Host 200 may, as one example, be apersonal computer 210 that executes application software 211, althoughother types of hosts 200 such as web servers, database engines, or othercomputers may also use the array 100 to store data. In FIG. 1 a networkinterface 212 is used by the host 200 to send commands and data to andfrom the storage array 100, such as via a computer network connection150 such as the Internet, private networks, or other networks. However,other types of connections between host 200 and array 100 are possible.

The storage array 100 includes a corresponding interface 102 enabling itto communicate with the host 200, an array controller 104, and at leasttwo different tiers of storage devices, including tier-zero 110 andtier-one 120 portions. In the specific embodiment illustrated, thetier-zero portion 110 is provided by physical storage devices having arelatively fast device access time, such as a Solid State Drive (SSD)array wherein the data storage devices are solid state integratedcircuits 116. The integrated circuits may be non-volatile flash or othersemiconductor memory technologies. SSD arrays provide extremely fastperformance to store persistent data.

The tier-one portion 120 may be provided by physical storage devicesthat have a somewhat slower device access time that the tier-zeroportion. The tier-1 portion may be provided, for example, by a Hard DiskDrive (HDD) array consisting of one or more electromagnetic spinningdisk type devices 125 with movable read-write heads. Compared to HDDs,SSDs are typically less susceptible to physical shock, are quieter, andhave much faster access time and latency. However, HDDs are typicallymuch less expensive on a per-byte basis. Most SSDs use NAND-based flashsemiconductors which can retain memory even without power. However,other solutions using volatile Random Access Memory (RAM) technologiescan also be used for situations where even faster performance is desiredbut where data persistence after power loss is needed or where batteriescan be used.

As shown, the tier-zero portion or SSD array 110 has its own diskcontroller 111 and optional RAID logic 112 functionality. Functions ofthe SSD array 110 may be provided by a large number of SSD drives 116-1,. . . , 116-n. Most of the SSD array is allocated to a main storagefunction, but in a preferred embodiment, a portion of the SSD array 110is reserved as a write cache extension (WCE) 115 whose purpose willbecome more evident in the discussion below. In other embodiments, theWCE may be located elsewhere in other “tiers”, but still implementedusing devices having a device access time that is faster than thetier-one portion.

The tier-one portion or HDD array 120 similarly has its own diskcontroller 121 and optional RAID logic portion 122. The tier-one portioncan be provided by a number of individual HDD disk drives, 125-1, . . ., 125-n.

The hybrid array controller 104 coordinates access to and between thetier-zero 110, tier-one 120 and a high-speed semiconductor memory usedas a local controller cache memory 106. These functions are typicallyimplemented as hybrid array software 105 but can also be implemented ingate arrays or hardwired circuits.

It should be understood that in certain embodiments, there may be morethan two tiers in the hybrid array 100, and that data may be managedbetween the multiple tiers analogous to the manner described herein fortwo tiers.

It should also be understood that the functions of controllers 111and/or 121 may be implemented inside the hybrid array controller 104.

FIG. 2 depicts the logical flow between a host application 211, thehybrid array software 105 and various components of the tier-zero 110and tier-one 120 storage including the write cache extension 115. At ahigh level, the host application 211 may issue two general types of hostapplication access patterns to the storage array 100.

One type of host application access pattern is a random access pattern,where the locations to be written are in a relatively random sequence.Such patterns exist when one or more storage commands result in accessto the storage array that are not sequential and/or access a relativelysmall amount of data, that is, somewhat less than a full RAID stripe.

A second type of host application access pattern is a “bulk” writeoperation 301 where a relatively large number of contiguous memorylocations are to be written. This may occur such as when an amount ofdata equal to or greater than a RAID stripe size is to be written.

It should be understood that other predetermined types of host accesspatterns to be detected may be predefined, and ranked according to theirrelative performance. Relative performance may depend on certainattributes of the access patterns, such as their typical speed ofcompletion.

If the operation is of the first type, where a relatively large amountof contiguous data is to be written, then the hybrid array software 105keeps an access frequency table 305 to help manage where the data willbe stored as follows. Pages in the array may be arranged, in say, fourdifferent frequency zones represented as very hot, hot, warm and cold.By frequency “zones”, it is meant that the controller 105 determines therelative demand for access to particular pages as compared to otherpages. As one example, very hot pages are accessed so frequently thatthey are cached in the storage controller cache memory 106. Hot pagesare moved to the SSD (tier-zero) 110 from the cache 106 when space isavailable. Cold pages are migrated to the HDD (tier-one) 120. Hot pagesmay also be traded to the SSD tier 110 in exchange for cold pages. Inthe absence of many hot pages, warm pages can be migrated to the SSDtier 120 or traded for cold pages by the same rules.

In addition, a background process running in the write controllersoftware 105 generally migrates pages and updates table 305 based upondemand frequency. For example, very hot pages are migrated to thecontroller cache 106, hot pages are migrated from the HDD tier 120 tothe SSD tier 110, and cold pages are migrated from the SSD tier 110 tothe HDD tier 120. Warm pages may be migrated back and forth between theSSD 110 and HDD 120 tiers depending upon demand for those as well.

As mentioned briefly above, an additional portion of the SSD tier 110(or other tier) is reserved as a write cache extension (WCE) 115. Thewrite cache extension portion 115 is used in a situation where the hostapplication 211 is requesting the first type of write operation, such asrandom writes. In this instance, the small, random writes that wouldotherwise be written to the HDD tier 120 as cold pages are first writtento the reserved space in the write cache extension 115. Also preferablywritten are the original logical block addresses (LBAs) for which therandom writes were originally targeted. This LBA information will beneeded when the write cache extension 115 is later flushed to itsdestination in the HDD tier 120.

The SSD write cache extension 115 may not be utilized in all instances.For example, it may not be utilized when the controller cache 106 itselfis not all that busy. This may occur when the array controller cache 106contains less than a certain amount of data. In this instance, therandom writes may be written to the HDD tier 120 directly (as indicatedby arrow 401). However, if the array controller cache 106 utilization isgreater than a certain amount (such as 70% as indicated by arrow 402),the write cache extension 115 may be enabled and begin absorbing theserandom writes that would otherwise be intended to be written directly toHDD tier 120 storage directly.

Once the contents of the write cache extension reach a certain size,such as at or near a RAID stripe size, they can be moved to the HDD tier120.

FIG. 3 is a flow diagram illustrating this in more detail. In a firststate 500, if an incoming write operation is not of the first accesstype, e.g., it is not a random burst, then processing proceeds to state501. This may be the case, for example, when the access is a blockaccess. In state 501, a direct write is then made to either thecontroller cache 106, the SSD tier 110 or the HDD tier 120, dependingupon whether the table 305 indicates the page being written to is veryhot, hot or cold.

If in state 500 the access is determined to be a random burst, thenstate 505 is entered in which a predetermined condition of the arraycache is detected. Such a condition may, for example, be whetherutilization of the array cache 106 is less than the predetermined amountof say 70%. If this condition is met, then processing returns to state501 where direct writes to the appropriate tier are enabled.

If, however, in state 505, the controller cache 106 is already storinggreater than the predetermined amount of data, then processing movesforward to state 506 where the random burst will be stored in the writecache extension 115 along with the logical block addresses.

Another optional process, indicated as starting at state 510,periodically determines whether the write cache extension 115 isexperiencing a certan condition, such as its utilization being less than70%. If this is not the case, then nothing is done. However, if thewrite cache extension is being utilized to more than 70% of itscapacity, then a state 512 is entered in which a test is made todetermine whether the write cache extension 115 has stored more than aRAID stripe size. If not, then nothing occurs. If it has, then the writecache extension 115 contents are written to the HDD tier in state 514.

It should now be understood that an information handling system andmethod implemented according to principles, described herein, will havemultiple tiers of storage, including at least a tier-zero portion and atier-one portion. A storage array controller migrates pages between thevarious tiers based on measured demand for a specific page's access as,for example, in a background process. If the storage command is detectedas a first type of relatively slow application access pattern, such as arequest for a relatively random write access, then under certainconditions in the storage cache, data associated with the random writeaccess is first stored in a write cache extension portion. Thecontroller then transfers data from the write cache extension portion toother tiers only after other conditions are met, such as if a number ofrandom writes have first been accumulated.

The write cache extension mode may be enabled all of the time, oralternatively, only when the main portion of a storage cache is utilizedby more than a certain predetermined amount.

In addition, data may be transferred from the write cache extensionportion only when the stored number of writes is at or near a RAIDstripe size. This helps with efficiency of writes to the tier-one.

Migration on demand between the various tiers can be based upon afrequency of read requests from external applications such as running ona remote host.

Metadata associated with the random writes can indicate a logical blockaddress for which the random writes are ultimately intended. Thisinformation is also stored in the write cache extension with the dataitself

The various storage tiers may be provided by hard disk drives, solidstate drives or by other data storage technologies.

It is also to be understood that this disclosure is not limited to theparticular apparatus, systems and methods described above, and as suchmay vary. One of ordinary skill in the art should understand that theterminology used herein is for the purpose of describing possibleaspects, embodiments and/or implementations only, and is not intended tolimit the scope of the present disclosure which will be limited only bythe appended claims.

It must also be noted that as used herein and in the appended claims,the singular forms “a,” “and,” and “the” may include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “an element” refers to one or several elements andreference to “a method of providing” includes reference to equivalentsteps and methods known to those skilled in the art, and so forth.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An information handling apparatus comprising: atier-0 storage portion; a tier-1 storage portion having a device accesstime that is slower than a device access time of the tier-0 storageportion; a write cache extension portion having a device access timethat is faster than a device access time of the tier-1 storage portion;a storage cache main portion; and a storage array controller that:receives a storage command for access to a page of data; detects if thestorage command is one of a series of random writes to the tier-1storage portion, and if so, sequentially write the series of randomwrites to a block of the write cache extension portion, and subsequentlywrite the block to the tier-1 storage portion when the block has reacheda particular size.
 2. The apparatus of claim 1 wherein the write cacheextension is part of the tier-0 portion.
 3. The apparatus of claim 1wherein the storage controller further stores meta-data associated witheach random write indicating an address in the tier-1 storage portionintended for the random write.
 4. The apparatus of claim 1 wherein thestorage array is configured to migrate, cache, and detect the storagecommand if an amount of data stored in the storage cache main portionexceeds a predetermined amount.
 5. The apparatus of claim 1 wherein datais transferred from the write cache extension portion to the tier-1portion after the amount of data stored via the first accesses patternis more than a predetermined amount.
 6. The apparatus of claim 1 whereinthe storage array sequentially writes the random writes to a block ofthe write cache extension portion if the series of random writes is lessthan a RAID stripe size used by the tier-1 storage portion.
 7. Theapparatus of claim 1 wherein the tier-1 storage portion comprises one ormore hard disk drives (HDDs).
 8. The apparatus of claim 1 wherein thetier-0 storage portion comprises one or more solid state drives (SDDs).9. The apparatus of claim 1 wherein the storage array controllermigrates pages of data between the tier-0, tier-1 and storage cacheportion by measuring a frequency of access requests, with the pagesorganized into a number of different frequency zones, such that pagesbelonging to a particular frequency zone are preferentially stored in aparticular one of the tier-0 portion, tier-1 portion, or the mainstorage cache portion.
 10. The apparatus of claim 9 wherein thefrequency zones include at least hot frequency zone and a cold frequencyzone, and the storage array controller further migrates hot pages to thetier-0 portion in exchange for relatively cold pages, and such coldpages are migrated to the tier-1 portion.
 11. The apparatus of claim 10wherein the frequency zones further include a warm frequency zone, andin the absence of hot pages, the storage array migrates warm pages tothe tier-1 portion by exchanging warm pages in the tier-1 portion withcold pages in the tier-0 portion.
 12. The apparatus of claim 1 whereinthe storage array controller is implemented as software code executingon a programmable data processor.
 13. A method for handling informationcomprising: receiving a storage command for access to a page of data ofa storage array, the storage array having a tier-0 storage portion, atier-1 storage portion, a storage cache main portion and a write cacheextension portion, wherein a device access time of the tier-1 storageportion is slower than a device access time of the tier-1 portion, andwhere a device access time of the write cache extension portion isfaster than the device access time of the tier-1 portion; anddetermining if a storage command is one of a series of random writes tothe tier-1 storage portion, and if so, sequentially write the series ofrandom writes to a block of the write cache extension portion, andsubsequently write the block to the tier-1 storage when the block hasreached a particular size.
 14. The method of claim 13 wherein the writecache extension portion is part of the tier-0 portion.
 15. The method ofclaim 13 additionally comprising: storing metadata associated with eachrandom write indicating an address in the tier-1 storage portionintended for the random writes.
 16. The method of claim 13 wherein thestorage array is configured to migrate, cache, and detect the storagecommand if an amount of data stored in the storage cache main portionexceeds a predetermined amount.
 17. The method of claim 13 wherein datais transferred from the write cache extension portion to the tier-1portion only after the amount of data stored via the predeterminedaccess pattern exceeds a predetermined amount.
 18. The method of claim13 wherein the storage array sequentially writes the random writes to ablock of the write cache extension portion if the series of randomwrites is less than a RAID stripe size used by the tier-1 storage. 19.The method of claim 13 wherein the tier-1 storage comprises one or morehard disk drives (HDDs).
 20. The method of claim 13 wherein the tier-0storage comprises one or more solid state drives (SDDs).
 21. The methodof claim 16 additionally comprising: migrating data between the tier-0,tier-1 and storage cache by measuring a frequency of access requests tothe storage array, with the pages organized into frequency zones, suchthat pages belonging to a particular frequency zone are preferentiallystored in one of the tier-0 portion, tier-1 portion, or main storagecache portion.
 22. The method of claim 21 wherein the frequency zonesfurther include at least hot frequency zone and a cold frequency zone,and the storage array controller further migrates hot pages to thetier-0 portion in exchange for relatively cold pages.
 23. The method ofclaim 22 wherein the frequency zones further include a warm frequencyzone, and in the absence of hot data, the storage array controllerfurther migrates warm data to tier-0 in exchange for cold data.
 24. Atangible, non-transitory, computer readable medium for storing computerexecutable instructions for providing a hybrid storage array function,with the computer executable instructions for: receiving a storagecommand for access to a page of data of a storage array, the storagearray having a tier-0 storage portion, a tier-1 storage portion, astorage cache main portion and a write cache extension portion, whereina device access time of the tier-1 storage portion is slower than adevice access time of the tier-1 portion, and where a device access timeof the write cache extension portion is faster than the device accesstime of the tier-1 portion; and determining if a storage command is oneof a series of random writes to the tier-1 storage portion, and if so,sequentially write the series of random writes to a block of the writecache extension portion, and subsequently write the block to the tier-1storage when the block has reached a particular size.